Question about a Python for loop

[member 428835]

Hi All

I'm learning python and have never seen a for loop positioned like this:

integers = [4, 5, 7]
strings = [str(integer) for integer in integers]

I've only ever seen for loops positioned at the beginning of lines. Is this usual? How can I know when this is okay? I think I would have tried something like this instead

integers = [4, 5, 7]
strings = range(len(integers))
for i in integers:
    strings(i) = [str(i)]

 

[Ibix]

It's called a "list comprehension" and is a fairly common construct, yes.

 

[Mark44]

It's called a "list comprehension" and is a fairly common construct, yes.

In Python... It's not a construct that's present in other, older languages, AFAIK. Possibly it's present in some of the newer languages.

 

[jedishrfu]

Here's more on the list comprehension:

https://en.wikipedia.org/wiki/List_...omprehension is a,of map and filter functions.

 

[PeterDonis]

I think I would have tried something like this instead

Your code has a syntax error in its last line. It also has two conceptual errors (three if you are using Python 3).

A really good piece of advice is to test any snippet of code in the interactive interpreter before posting it. Even better, post the actual interpreter session:

>>> integers = [4, 5, 7]
>>> strings = range(len(integers))
>>> for i in integers:
...     strings(i) = [str(i)]
...
  File "<stdin>", line 2
SyntaxError: cannot assign to function call
>>> for i in integers:
...     strings[i] = [str(i)]
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: 'range' object does not support item assignment
>>> strings = list(range(len(integers)))
>>> for i in integers:
...     strings[i] = [str(i)]
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
IndexError: list assignment index out of range
>>> for i in integers:
...     print(i)
...     strings[i] = [str(i)]
...
4
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
IndexError: list assignment index out of range
>>> for idx, i in enumerate(integers):
...     strings[idx] = [str(i)]
...
>>> strings
[['4'], ['5'], ['7']]
>>> for idx, i in enumerate(integers):
...     strings[idx] = str(i)
...
>>> strings
['4', '5', '7']

Only the very last is actually what is intended.